Re: [PATCH v6 10/16] mm: replace vm_lock and detached flag with a reference count

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Dec 18, 2024 at 01:53:17PM -0800, Suren Baghdasaryan wrote:

> Ah, ok I see now. I completely misunderstood what for_each_vma_range()
> was doing.
> 
> Then I think vma_start_write() should remain inside
> vms_gather_munmap_vmas() and all vmas in mas_detach should be

No, it must not. You really are not modifying anything yet (except the
split, which we've already noted mark write themselves).

> write-locked, even the ones we are not modifying. Otherwise what would
> prevent the race I mentioned before?
> 
> __mmap_region
>     __mmap_prepare
>         vms_gather_munmap_vmas // adds vmas to be unmapped into mas_detach,
>                                                       // some locked
> by __split_vma(), some not locked
> 
>                                      lock_vma_under_rcu()
>                                          vma = mas_walk // finds
> unlocked vma also in mas_detach
>                                          vma_start_read(vma) //
> succeeds since vma is not locked
>                                          // vma->detached, vm_start,
> vm_end checks pass
>                                      // vma is successfully read-locked
> 
>        vms_clean_up_area(mas_detach)
>             vms_clear_ptes
>                                      // steps on a cleared PTE

So here we have the added complexity that the vma is not unhooked at
all. Is there anything that would prevent a concurrent gup_fast() from
doing the same -- touch a cleared PTE?

AFAICT two threads, one doing overlapping mmap() and the other doing
gup_fast() can result in exactly this scenario.

If we don't care about the GUP case, when I'm thinking we should not
care about the lockless RCU case either.

>     __mmap_new_vma
>         vma_set_range // installs new vma in the range
>     __mmap_complete
>         vms_complete_munmap_vmas // vmas are write-locked and detached
> but it's too late

But at this point that old vma really is unhooked, and the
vma_write_start() here will ensure readers are gone and it will clear
PTEs *again*.